The present invention relates to a method and apparatus which implements a rotary electric actuator for controlling a variable geometry turbocharger, and includes a second rotary electric actuator for controlling an exhaust gas recirculation valve in an exhaust gas recirculation system for an engine.
Much public literature describes the use of alternative charge air handling in turbocharging concepts to drive and control cooled EGR (exhaust gas recirculation), as a primary means for NOx reduction in automotive and truck engines. One of the more popular approaches is to use a single stage VGT (variable geometry turbocharger), in combination with an EGR (exhaust gas recirculation) circuit to achieve the desired ratio of EGR rate and fresh air/fuel ratio, under both transient and steady state operations. In a typical arrangement, the EGR circuit includes a valve, cooler, and tubing connecting the exhaust side of the engine (manifold or turbine) with the intake side of the engine (intake manifold or intake piping). The EGR valve may be an on/off or modulating (proportional) type valve to regulate EGR flow, and it may be mounted on the turbine, exhaust manifold, between the exhaust manifold and cooler, or on the downstream (cool) side of the EGR cooler. Other derivatives of this common arrangement may include a mixing device at the point of EGR gas entry to the inlet manifold and/or a venturi device to encourage a negative pressure differential across the engine as required to drive EGR flow from the exhaust side to the intake side of the engine.
In many diesel engines, particularly larger engines under low speed and moderate-to-high load operation, the turbocharger match is relatively efficient. Therefore, intake side (compressor-out) pressure (or boost) levels usually exceed exhaust side (turbine-in) pressure (or boost) and a so-called positive pressure differential exists across the engine under a wide range of steady state or near steady state operating conditions. To drive EGR from the exhaust to the intake side of the engine, a negative pressure differential (exhaust pressure greater than intake pressure) must be imposed on all or part of the engine gas flow. In many proposed EGR arrangements, the VGT performs the primary role of reversing the pressure differential across the engine. However, during the breathing portion of the four stroke cycle, engine pumping parasitics and BSFC (brake specific fuel consumption) are increased. As the turbine vanes of the VGT are moved to a more closed position, turbine and compressor wheel speeds increase, as do overall boost levels. Depending on the contour of the turbine and the compressor efficiency maps versus flow and boost, turbine and compressor efficiencies will eventually begin to deteriorate as wheel speeds and boost increase. Turbine-in pressure will ultimately exceed compressor-out pressure, thereby creating the necessary overall negative pressure differential across the engine, conducive for EGR to flow from the exhaust manifold to the intake manifold. Manifold gas dynamics and associated pressure pulses will enable some amount of EGR to begin to flow, even though the cycle average to pressure differential across the engine is slightly positive. This is the subject of many proposed EGR configurations. If large EGR flow rate percentages are required, the cycle average pressure differential will also become negative.
Under most scenarios involving EGR, EGR gas displaces some of the fresh, boosted air normally ingested into the engine. To preserve reasonable air/fuel ratios for combustion efficiency and completeness while accommodating EGR in a given engine displacement, fresh air intake charge density must be increased accordingly. This can be achieved via higher boost levels, and to a lesser degree, by improved charge air cooling. Hence, the moving of VGT vanes to a more closed position (relative to non-EGR operation) simultaneously achieves the requirement of higher fresh air boost levels required to maintain adequate fresh air/fuel ratios (and oxygen content) for efficient combustion and low particulate emissions.
Boost levels must be increased relative to the non-EGR operation in order to preserve air/fuel ratios, unless the combustion system has been developed to achieve performance and emissions objectives under lower brake specific air consumption (BSAC) rates.
For a given engine, a desired engine rating or torque curve can be described by the relationship of torque and horsepower at a specific engine speed. To produce the desired engine torque curve while using EGR to reduce the level of NOx, the degree of VGT vane position changes for a given speed and load (i.e. torque). The degree of vane closure required to drive the necessary amount of VGR can be characterized as a function of the required negative pressure differential across the engine versus engine speed. One consequence of using a VGT to drive EGR is that the sensitivity of vane position and its associated NOx level changes with the required negative pressure differential across the engine. The vane position sensitivity to NOx typically increases with lower engine speeds.
Under many operating conditions, adequate air/fuel ratios can be achieved with operating the EGR valve in a full open position and setting the VGT vane position as required to provide the necessary negative pressure differential across the engine to provide the desired amount of EGR flow. However, under certain operation conditions, the proper EGR rate and air/fuel ratio cannot be achieved with a fully open EGR valve.
In the past, VGT position and EGR valve position have been controlled by pneumatic actuators. However, this type of control is insufficient to achieve accurate EGR control under all modes of normal and regulated operation, and over the life of the engine.
The present invention overcomes the above-referenced shortcomings of prior art EGR systems by providing a rotary electric actuator (REA) operatively connected to the variable geometry turbocharger (VGT) for controlling exhaust gas recirculation. In a preferred embodiment, a second rotary electric actuator is operatively connected to the position-adjustable proportional EGR valve for further EGR control.
More specifically, the present invention provides a method of controlling an internal combustion engine, wherein the engine includes a variable flow exhaust gas recirculation (EGR) system. The method includes: a) providing a variable geometry turbocharger (VGT) in communication with the EGR system for controlling exhaust gas recirculation; and b) varying the geometry of the VGT by adjustably controlling a rotary electric actuator (REA) which is operatively connected to the VGT.
In a preferred embodiment, the method further includes providing a position-adjustable proportional flow EGR valve as part of the EGR system, and varying the position of the proportional flow EGR valve by adjustably controlling a second rotary electric actuator (REA) operatively connected to the position-adjustable proportional flow EGR valve.
Another aspect of the invention contemplates an engine including an intake manifold and a variable geometry turbocharger (VGT) operatively connected to the intake manifold. An exhaust gas recirculation (EGR) system is provided in communication with the intake manifold, and includes a position-adjustable proportional flow EGR valve. A first rotary electric actuator (REA) is operatively connected to the VGT for controlling recirculation of exhaust gases. A second rotary electric actuator (REA) is operatively connected to the EGR valve to enhance the control of recirculation of exhaust gases.
Accordingly, an object of the invention is to provide an improved method and apparatus for controlling exhaust gas recirculation by providing a rotary electric actuator (REA) to adjustably control a variable geometry turbocharger (VGT).
A further object of the invention is to provide first and second rotary electric actuators to control a variable geometry turbocharger (VGT) and a proportional flow EGR valve, respectively, in an engine exhaust gas recirculation (EGR) system.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.